Let’s resolve into factors

x(x + 3)(x + 6)(x + 9) + 56

x(x + 9)(x + 3)(x + 6) + 56

= (x² + 9x) (x² + 6x + 3x + 18) + 56

= (x² + 9x) (x² + 9x + 18) + 56

Let x² + 9x = a

= a (a + 18) + 56

= a² + 18a + 56

= a² + 14a + 4a + 56

=a(a + 14) + 4(a + 14)

= (a + 4)(a + 14)

Put back the values to get,

= (x² + 9x + 4)( x² + 9x + 14)

= (x² + 9x + 4)( x² + 7x + 2x + 14)

= (x² + 9x + 4)[( x(x + 7) + 2(x + 7)]

= (x² + 9x + 4)(x + 2)(x + 7)